This invention relates to restraining devices for preventing movement of a filter cloth, also to filtering machines that employ a movable filter cloth and a restraining device for this cloth, and to a method of filtering a material using a horizontal vacuum filter.
Horizontal vacuum filtering machines employing a movable belt filter have come into wide spread use for precise, continuous liquid/solid separation, cake washing and drying. A horizontal belt filter is described in U.S. Pat. No. 3,870,641 which issued Mar. 11, 1975 to H. G. Pierson. This machine employs an endless belt of filter cloth, this belt having an upper reach that extends over a series of vacuum trays. Each tray is connected by vacuum pipes to a vacuum pump. At the end of the upper reach, the belt passes over a movable, terminal discharge roller which can be moved by rams to an extended position. On the return reach of the belt, it passes through a belt washer wherein jets of water remove traces of any filter cake. The belt then passes over a drive roller and a drive contact roller, each of which is provided with a ratchet so that they can rotate only in the forward direction. It is important to prevent the lower reach of the filter belt from being pulled back by the outward movement of the discharge roller.
In the current versions of the above filtering machine, the vacuum is removed from the vacuum trays before the discharge roller is moved. The discharge roller acts now as the drive roller and its movement to the extended position causes the filter cloth to advance over the series of vacuum trays, one tray at a time. Vacuum is applied to the vacuum trays again before the discharge roller is caused to retract.
Although the method of restraining backward movement of the filter cloth described in the above-mentioned U.S. patent is generally satisfactory, it can fail under some filtration conditions that leave the filter cloth surface soiled. If the filter cloth is still soiled when it is passing over the drive roller, the cloth can slip causing the filter to operate in an unpredictable manner and possibly preventing operation of the filtering machine.
Although modifications have been attempted to the aforementioned driver roller and drive contact roller in these horizontal vacuum filters, these modifications have not been very successful. One such previous modification involved trapping the filter cloth between a fixed member and a movable member which applied pressure generated by an air actuated cylinder or similar device. This known clamp roll can cause damage to the filter fabric because it applies a relatively high point force to the filter cloth.
It is an object of the present invention to provide a relatively simple, inexpensive means for preventing movement of a filter cloth in a vacuum filtering machine when this is required.
It is a further object of this invention to provide a vacuum clamp capable of preventing movement of an elongate filter cloth, which clamp has a perforated support surface for supporting the filter cloth and a sealing sheet that extends over this support surface.